There are many internal air flows within a gas turbine. Some of these flows are oil laden. Examples of these oil laden flows are the vent lines from bearing compartments and gearbox housings.
For environmental reasons, and to reduce oil consumption, it is necessary to include some form of de-oiling in these oil laden flow lines.
A typical de-oiler consists of a rotating element that contains a porous medium. The rotating element is located within a stationary housing with appropriate seals. Oily air is admitted into the housing whereupon rotation of the rotating element causes the oil droplets to be centrifuged to the outer radius of the housing where they are collected as coalesced oil.
Air, being much less dense than oil, is able to flow though the rotating porous medium with much less impediment. This flow path of air can either be radially towards the centre of the de-oiler, or axially across the de-oiler. De-oiled air is collected and passed through the housing for use or disposal.
However, in such de-oiler arrangements, there is an appreciable pressure drop caused by the air having to pass through the porous medium. A further cause of pressure drop may result from the de-oiled air being exhausted at a smaller radius than that at which the oily air is admitted to the de-oiler.
This pressure drop can adversely affect the oil sealing performance of the assemblies from which the oil air is being exhausted. For example, the high back-pressure caused by the de-oiler may increase the pressure within a bearing chamber and thereby cause oil leaks through rotating seals.